1. Field
Exemplary embodiments of the present disclosure relate to an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunctional device having at least two of the foregoing capabilities, and a fixing device employed in the image forming apparatus.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction apparatuses having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
For example, a fixing device like that described in JP-2008-158482-A or JP-2007-334205-A includes a substantially-pipe-shaped metal member (opposed member) to effectively heat an endless fixing belt serving as a fixing member to shorten a warm-up time or a time to first print (hereinafter also “first print time”). Specifically, the metal member is provided inside a loop formed by the endless fixing belt so as to face a portion or the entire of the inner circumferential surface of the fixing belt. The metal member is heated by a built-in or external heater so as to heat the fixing belt. A pressing roller presses against the outer circumferential surface of the fixing belt at a position corresponding to the location of the metal member inside the loop formed by the fixing belt to form a nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. As the recording medium bearing the toner image passes through the nip, the fixing belt and the pressing roller apply heat and pressure to the recording medium to fix the toner image on the recording medium. For such a fixing device, when the pressing roller is rotated by a driving unit, the fixing belt in pressure contact with the pressing roller at the nip is rotated by friction resistance in accordance with the rotation of the pressing roller.
Alternatively, JP-2004-021079-A proposes an on-demand fixing device employing a ceramic heater to prevent overheating of a fixing belt when the fixing belt slips. The on-demand fixing device includes two temperature detectors to detect the temperature of the ceramic heater and stops heating of the ceramic heater when the difference between temperatures detected by the temperature detectors is equal to or greater than a predetermined threshold.
For the above-described fixing devices like those described in JP-2008-158482-A and JP-2007-334205-A, when a sufficient driving force is not transmitted from the pressing roller to the fixing belt, a slip (rotation failure) of the fixing belt may occur. Such a slip of the fixing belt may cause overheating at a portion of the fixing belt, thermally damaging the fixing belt. In particular, since the fixing device is highly efficient in heating the fixing belt, such a problem is not negligible. Further, the above-described fixing devices like those described in JP-2008-158482-A and JP-2007-334205-A employ a halogen heater as the heating unit. Such a configuration has a limitation in application of a technique of JP-2004-021079-A using the two temperature detectors to detect the temperature of the ceramic heater.